That's the way Blacklist ended. It didn't really 'end' in that sense. It just suddenly stopped. And that was it.
@240B I suspect they leave it "open" at the end so they can bring it back if what replaces it tanks. Probably with a few new lead actors/actresses the producers can pay less,and a fresh approach to the same problems.
And,frankly,after several years of watching the same characters on a tv show,lots of the viewers get bored and go look for something else to watch.
The one exception I can think of is Shameless. I miss the hell out of it,but it HAD to end when it did because it was about a family of children surviving despite their father and mother,and eventually,children grow up,get married,and move out. It's the natural order of things.
Of course,"Frank" will live forever in MY mind as living proof that there is really no such thing as a level too low for some people to slither under. Frank was probably THE most despicable character to appear on a tv series.